PILOT TESTING OF UPFLOW CONTINUOUS BACKWASH FILTERS …€¦ · lbs/1000 ft3/d at an average...

PILOT TESTING OF UPFLOW CONTINUOUS BACKWASH FILTERS FOR TERTIARY DENITRIFICATION AND PHOSPHORUS REMOVAL

P. Schauer,* , R. Rectanus,* C. deBarbadillo,*

D. Barton,** R. Gebbia,** B. Boyd,*** M. McGehee*** *Black & Veatch

18310 Montgomery Village Ave, Suite 500 Gaithersburg, MD 20879

**City of Hagerstown, Hagerstown, MD

***Parkson Corporation, Fort Lauderdale, FL ABSATRACT Pilot testing of upflow continuous backwash filter (UCB) for tertiary denitrification and phosphorus removal was conducted at the Hagerstown wastewater treatment plant. The pilot testing was conducted to determine denitrification performance capabilities of the upflow continuous backwash filters under cold weather operation and low effluent total nitrogen (TN) and total phosphorus limits. As part of a state-wide strategy for meeting the nutrient reduction goals of the Chesapeake 2000 Agreement with the U.S. Environmental Protection Agency and the other Chesapeake Bay watershed states, the State of Maryland is requiring all WWTPs > 0.5 mgd be upgraded to meet “limit of technology” (LOT) effluent standards of 3 mg/L TN and 0.3 mg/L TP on an annual average basis. The testing program included operation over a range of hydraulic and nitrate mass loading rates, moderate and cold weather operation, with and without chemical phosphorus removal; and testing the response of the filters to nitrate, solids spikes and wet weather flows. At hydraulic loading rates up to 4.0 gpm/ft2, the filter consistently reduced NO3-N from 5-8 mg/L to

INTRODUCTION Downflow, deep bed denitrification filters have been successfully used for 25 years to meet total nitrogen (TN) limits to as low as 3 mg/L. Most of this experience has been in the southeastern US, with relatively warm wastewater temperatures and with adequate phosphorus in the filter influent wastewater to support the growth of denitrifying bacteria. Upflow continuous backwash (UCB) filters have been used for just over 10 years in a limited number of denitrification applications in the US and Europe, but currently do not have operating history for meeting very low TN limits. With renewed interest in UCB filters for tertiary denitrification, limited cold weather operating data, and the requirement in some areas to meet very low TP limits simultaneously, there was a need to further assess UCB filter denitrification performance capabilities.

As part of a state-wide strategy for meeting the nutrient reduction goals of the Chesapeake 2000 Agreement with the U.S. Environmental Protection Agency and the other Chesapeake Bay watershed states, the State of Maryland is requiring all WWTPs > 0.5 mgd be upgraded to meet “limit of technology” (LOT) effluent standards of 3 mg/L TN and 0.3 mg/L TP on an annual basis. The City of Hagerstown pilot tested UCB filters for tertiary denitrification and phosphorus removal to meet these limits. Hagerstown is interested in UCB filters because of the simplicity of operation, small footprint, elimination of a clearwell and mudwell, and the ability to tolerate high solids and flow spikes during wet weather events. Testing was conducted through the winter months to develop design criteria for cold weather operating conditions in western Maryland. REVIEW OF PREVIOUS UCB FILTER PERFORMANCE DATA Full scale testing of UCB filters at three small WWTPs in Puerto Rico achieved NO3-N removal rates of 15 to 35 lbs/1,000 ft3/d at average hydraulic loading rates of 1.1 to 2 gpm/ft2 and methanol dosage ratios of 3.3 mg/mg NO3-N. In pilot testing conducted by the University of Florida in Gainesville, FL and at Aberdeen, MD (Koopman et al., 1990), hydraulic loading rates ranging from 1.4 to 4.6 gpm/ft2 were tested at influent NO3-N concentrations of 9 to 12 mg/L. When methanol dosage ratios were below 3.3, effluent NO3-N concentrations of 2 to 6 mg/L were achieved, but when methanol dosage was increased an effluent NO3-N of < 1 mg/L was achieved. These results were promising but reflect warm weather operation (temperatures between 18°C and 30°C) at elevated methanol dosage rates. Full scale data from a UCB in Sweden showed a NO3-N reduction from 7.3 – 20 mg/L to 0.5 – 2.2 mg/L (Hultman et al., 1994). Testing showed that performance was impacted by temperature and hydraulic loading. The results indicated that the phosphorus concentration into the filter may limit denitrification at values less than 0.1 mg/L soluble phosphorus.

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Copyright 2006 Water Environment Foundation. All Rights Reserved©

Full scale data from a UCB filter in the Netherlands showed NO3-N removals averaging 70 lbs/1000 ft3/d at an average hydraulic loading rate of 4.1 gpm/ft2 (Kramer et al., 2003), but the filter received dry weather flows only and operated at an elevated methanol dosage ratio of 3.3. KEY ISSUES FOR MEETING “LIMITS OF TECHNOLOGY” IN MARYLAND In addition to the need to supplement available warm weather data for UCB filters with cold weather operating results, there are several issues associated with the application of this technology for LOT limits. First, meeting 3 mg/L TN means that the filter must consistently meet < 1 mg/L NOx-N. Denitrifying to very low NOx-N concentrations normally requires lower loading rates and/or high methanol dosages, which is undesirable from the perspective of meeting low effluent BOD limits and minimizing chemical costs. Second, the concurrent removal of TP by chemical precipitation ahead of the filters is complicated by the need for sufficient residual bio-available phosphorus to sustain biological growth in the filter. To accomplish both goals (TN

HAGERSTOWN ENHANCED NUTRIENT REMOVAL PILOT PLANTPARKSON CORPORATION DYNASAND ® FILTER

LEGEND

Composit Sampler Temperature MeterGrab Sample Tap pH MeterTurbidity Meter Oxidation/Reduction Potential MeterDissolved Oxygen Meter Differencial Pressure Transmitter

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Figure 1. Flow Schematic of Pilot Test The DynaSand filter is an upflow continuous backwash filter. The pilot unit had an effective media depth of 80 inches and a cross-sectional area of 10.7 ft2. A 1.65 mm sub-angular media sand was used. Influent wastewater enters the filter bed through radials located a the bottom of the filter. The flow moves up through the downward-moving sand bed and flows over a weir at the top of the filter. Compressed air is introduced through an air lift tube extending to the conical bottom of the filter creating an air lift pump that lifts the sand at the bottom of the filter up the center column where it enters the reject bowl/washer assembly. The sand settles down a baffled washer section and is deposited on top of the filter bed. The reject bowl has a 4 5/8-inch overflow weir set at a lower elevation than the filter effluent weir. A portion of the filtrate flows countercurrent to the sand in the washer assembly and over the reject weir. The agitation in the air lift and the washer section help to loosen and carry away solids with the reject water. A schematic of the DynaSand filter is shown in Figure 2.

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Figure 2. Schematic of Pilot DynaSand Filter DATA COLLECTION AND RESULTS The denitrification filter pilot testing was conducted in six phases over a period of eighteen weeks from November 17, 2004 through March 18, 2005. Each phase was designed to test the denitrification capabilities of the filter under a variety of hydraulic, nitrogen and phosphorus loading conditions. The test plan was designed to operate the filter under a number of conditions to determine the limits of performance. The six testing phases are described below.

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1. System Startup 2. Constant Rate Hydraulic Loading 3. Phosphorus Precipitation 4. Diurnal Flow Rate 5. Increased Nitrate Loading 6. Hydraulic Loading Spike

As a convention for presentation of data graphs in this report, online monitoring data is presented as a continuous line and laboratory data is presented as individual data points. Although the ChemScan analyzer operated continuously, a cycle time of approximately 20 minutes was required to complete all of the analyses required for both the filter influent and filter effluent sample streams, resulting in three or four discrete measurements per hour. For clarity, transient spikes in the online monitoring data resulting from mechanical failures, equipment maintenance, or loading changes have been removed from the database used to generate the plots. The data points for 24-hour composite laboratory samples are placed at the midpoint of the 24-hour time period during which the samples were collected. Phase 1 – Pilot Startup The pilot plant was placed into service on November 5, 2004 with a hydraulic loading rate of 2.0 gpm/ft2. During the first two weeks of operation, denitrification was inhibited by widely fluctuating dissolved oxygen (DO) concentrations in the filter influent, varying from 2 to over 7 mg/L. With such high dissolved oxygen present, the facultative heterotrophic bacteria that developed in the filter consumed most of the methanol aerobically and accomplished little, if any, anoxic denitrification. The filter feed pump was relocated resulting in diurnal pattern of the DO concentration that normally varied between 0 and 4 mg/L. Once the filter influent DO was brought under control on November 18, it took about a week for the pilot filter to achieve full denitrification (Figure 3). It should be noted that even though the hydraulic loading rate on the filter was constant, there was significant diurnal variation in the filter influent NOx-N and DO concentrations that is reflective of actual plant secondary effluent characteristics.

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Figure 3NOx and DO During Startup

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Phase 2 – Constant Rate Hydraulic Loading Hydraulic loading rate is an important design criterion because it defines the empty bed detention time (EBDT), or contact time. The pilot filter was started up at a constant hydraulic loading rate of 2 gpm/ft2 and stepped up through a series of increases in hydraulic loading. Figure 4 shows filter influent and effluent NOx-N concentrations over a range of hydraulic loading rates from 2 gpm/ft2 to 4 gpm/ft2. Effluent NOx-N concentrations consistently remained below 1 mg/L up to a loading rate of 4 gpm/ft2. Denitrification performance began to deteriorate at 4 gpm/ft2 loading. It is unclear whether this reduction in treatment efficiency resulted from the increased hydraulic loading rate or from very low influent OP concentrations observed during that period.

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Figure 4NOx-N Concentrations During Constant Hydraulic Loading

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Phase 3 – Phosphorus Precipitation A key objective of the pilot testing was to determine if additional phosphorus removal could be achieved with chemical precipitation immediately ahead of the denitrification filter if needed to meet the Maryland ENR total phosphorus goal of 0.3 mg/L. The testing results indicate that when the soluble ortho-phosphorus concentration entering the filter from late November through December 7 was less than 0.2 mg/L, the ENR goal of 0.3 mg/L TP was generally met without the need for any additional chemical phosphorus precipitation. However, when influent ortho-phosphorus concentrations rose above about 0.2 mg/L, additional phosphorus removal was required. Specific testing was conducted to determine whether phosphorus precipitation and denitrification could be accomplished simultaneously to meet the effluent limits of 3 mg/L TN and 0.3 mg/L TP. A secondary objective of this phase of testing was to determine if a reaction tank would be required upstream from the filter to promote flocculation of the chemically precipitated phosphorus. The chemical phosphorus removal testing was run concurrent with testing at constant rate hydraulic loading and was divided into two sub-phases: (1) chemical precipitation of phosphorus using a reaction tank, and (2), chemical precipitation of phosphorus using direct chemical dosing. During most of the testing period, the WWTP BNR process performed so well for phosphorus removal that phosphorus concentrations in the WWTP secondary effluent were below the 0.3

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mg/L ENR limit. In order to test the feasibility of chemical precipitation for phosphorus removal, it was necessary to increase the filter influent phosphorus concentration. Phosphoric acid was dosed to the pilot filter influent during portions of testing to increase the filter influent phosphorus concentration. Ferric chloride was used to chemically precipitate phosphorus. Ferric chloride dosages were automatically adjusted using flow and the influent OP concentration as reported by the online monitoring system. The control system determined the ferric chloride feed rate based on a 1:1 molar ratio of ferric chloride to OP. Chemical phosphorus removal was first tested incorporating approximately 5 minutes of retention time upstream of the filter following the addition and initial mixing of the ferric chloride. The reaction detention time was achieved using 60 feet of 8-inch PVC piping located between the trailer and the filter feed riser pipe. During this phase of the test, the filter hydraulic loading rate was maintained at 3.0 gpm/ft2 and phosphoric acid was dosed at a rate of 1.0 mg/L. At this hydraulic loading rate, the available contact time in the reaction “tank” was 4 minutes, 50 seconds. The pilot filter was operated with the reaction “tank” in service for eight days. After this, the reaction “tank” was removed, but all other operational setpoints remained the same. Operating conditions without the reaction “tank” were held constant for seven days, when the hydraulic loading rate was increased to 3.5 gpm/ft2. Pilot influent and effluent phosphorus concentrations during this testing period are shown in Figure 5.

Figure 5Phosphorus Removal with and without Reaction Tank

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Reaction Tank No Reaction Tank

Loading rate increased from 3.0 gpm/ft2 to 3.5 gpm/ft2FeCl3

Pump fault

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As indicated in Figure 5, low effluent phosphorus concentrations were achieved equally well both with and without the reaction tank. The effluent phosphorus increase on January 13 was due to disruption of the filter operation for removal of the reaction “tank” and flushing of accumulated solids from both the filter feed and effluent piping. Effluent OP remained low following January 13, however, the flushing of the effluent pipeline between the filter and the trailer resulted in the flushing of suspended solids into the filter and elevated effluent TP concentrations for several days. The effluent TP concentrations came down as effluent TSS concentrations restabilized. The hydraulic loading rate was increased from 3.0 to 3.5 gpm/ft2 on January 20th, and was further increased to 4.0 gpm/ft2 on January 24th. Neither of these loading changes resulted in any increase in effluent total phosphorus concentration. During this phase of testing, the target ferric chloride dosage corresponded to 1:1 molar ratio of iron applied to OP removed. The results show that the overall dosage ratio from January 6 though January 27 was 1.12 moles Fe3+ applied per mole OP removed (2.01 mg Fe3+/ mg OP removed). The removal efficiency did not deteriorate when the reaction tank was removed from the line as can be seen in Table 1. It is noted these ratios are significantly lower than those reported in literature (Luedecke et al, 1988; WEP 1998). Typical values to reach an effluent OP concentration of 0.3 mg/L can be as high as 7 mg Fe3+/ mg OP removed (or about 3.9 moles Fe3+/mole OP removed). This ratio increases as target effluent phosphorus concentration is reduced.

Table 1 Ferric Chloride Dosage Use for Phosphorus Removal

Fe3+ Applied / OP Removed Time Period mg/mg mol/mol

Reaction Tank 1/5/05 – 1/13/05 2.24 1.24

No Reaction Tank 1/13/05 – 1/27/05 1.93 1.07

Combined Averages 2.01 1.12

The dosage ratios shown in Table 1 are actually slightly low because they do not account for some of the phosphorus being removed through biomass growth in the filters. On January 9th, the ferric chloride was not dosed to the system due to an equipment malfunction. The OP concentration was reduced by 0.19 mg/L without the assistance of the ferric chloride. If this value is assumed to be the biological phosphorus uptake the ratio of ferric chloride to phosphorus precipitation can be recalculated with the biological uptake accounted for. The resulting ratio (2.88 weight ratio and 1.60 molar ratio) is still much lower than anticipated values.

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Phase 4 – Diurnal Flow Rate During dry weather, the wastewater flow rate to the Hagerstown WWTP follows a repeatable diurnal pattern that is characteristic of the service area and the customers it serves. Because the plant does not include flow equalization basins, the diurnal flow variation carries through the plant and will result in changes in the effective hydraulic loading rate on the filters. The impact of dry weather diurnal flow variation on filter performance was evaluated. The Hagerstown WWTP secondary effluent also shows significant diurnal variations in concentration for nearly all water quality parameters. The use of the online monitoring equipment used in the pilot testing allowed documentation of the diurnal effluent quality variations and control of treatment chemical feed rates in response to the changes in secondary effluent quality. Nutrient concentrations were observed to vary by more than 100% over the course of a day. To assess the filter performance under normal dry weather operating conditions, the pilot plant was operated with a diurnal variation in flow rate from February 4 through 17. The diurnal flow pattern was developed using the Hagerstown WWTP dry weather effluent flow data. Over the course of a day, flow rates varied from 63% to 125% of the average flow. At the average hydraulic loading rate of 3.5 gpm/ft2, these flow variations corresponded to effective loading rate variations from 2.2 gpm/ft2 to 4.4 gpm/ft2. The flow rate, NO3-N concentration, and NO3-N mass loading as a function of the time of day are shown in Figure 6.

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Figure 6Diurnal Variation in Flow,

NOx-N Concentration, and NOx-N Loading

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Because the diurnal variations for NO3-N concentration and hydraulic loading are out of phase, the diurnal variation in NO3-N mass loading is dampened. The minimum NO3-N concentration occurs about the same time that the flow rate is peaking for the day. The minimum mass loading to the filter occurs during the period of minimum flow from approximately 3 am to 7 am. The maximum mass loading occurs during a period of relatively high sustained flow in the evening from 5 pm until about midnight. The pilot plant performed very well under the diurnal flow conditions and the effluent met the State of Maryland ENR goals for both nitrogen and phosphorus for the duration of the Phase 4 testing as shown in Figure 7.

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Figure 7Pilot Performance During Operation with Diurnal Flow Variations

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Phase 5 – Increased Nitrate Loading To determine the maximum nitrate removal capability of the filter, nitrate loadings were increased by adding sodium nitrate to the filter influent. The NOx-N loading and removal during this period is shown in Figure 8.

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Figure 8NOx-N Mass Loading and Removal at High Nitrate Loading Conditions

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Figure 8 suggests that the pilot is capable of removing up to 120 lbs NOx-N/1000ft3/d. It is noted that during this period, methanol was overdosed because the feed forward methanol dosage calculation is based on the assumption that all of NO3-N, NO2-N and DO being removed. The filter effluent showed breakthrough of both phosphorus and methanol (as indicated by an increase in COD). Therefore, the limiting factor under these loading conditions was likely the amount of biomass that could be supported by the system under the specific operating conditions. Phase 6 – Hydraulic Loading Spike One of the goals of the pilot testing was to evaluate the response of the filter to high flows similar to what would be experienced by the WWTP during wet weather flow conditions. Of particular interest are the filter’s hydraulic capacity, hydraulic loading/headloss characteristics, suspended solids removal capabilities, and the recovery time to re-establish denitrification after the high flow events. It is not expected that the filters be capable of maintaining denitrification during a transient peak flow event, but we were interested in determining how the denitrification performance would be affected during and after such an event. Due to hydraulic limitations in the influent and effluent delivery systems for the pilot filter the testing was limited to the equivalent of approximately 6 gpm/ft2.

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The Phase 6 testing was initiated at a loading rate of 4.8 gpm/ft2. After one day of operation, the hydraulic loading rate was increased to 6 gpm/ft2. Methanol feed was maintained through an initial peak flow period at 6 gpm/ft2. The filter was then operated over a series of daily cycles at 6.0 gpm/ft2 with no methanol addition followed by a day of operation at an average hydraulic loading rate of 3.5 gpm/ft2 with methanol addition. This testing sequence provided information on the filter performance during high flow events in two ways. First, operation with methanol addition provided information on the denitrification capability at high flow rates. Second, when the pilot was operated without methanol during the period of high hydraulic loading followed by a return to normal diurnal conditions with methanol, the ability of the system to recover and return to denitrification mode was examined. The results from the first three days of operation under this testing sequence are shown in Figure 9. The pilot filter maintained denitrification to an effluent NOx-N concentration of less than 1 mg/L at hydraulic loading rates of 4.8 and 6.0 gpm/ft2. The 6 gpm/ft2 hydraulic loading rate corresponds to an empty bed detention time of 8.3 minutes. The filter influent and effluent COD data also included in Figure 9 show that there was a net removal of COD through the filter, indicating that there was no methanol breakthrough to the effluent during this period.

Figure 9Denitrification under Peak Hydraulic Loading Conditions

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At full scale operation, it is likely that methanol feed to the filters will be shut down during peak wet weather flow conditions. To evaluate the ability of the filter to recover denitrification after such an event, the methanol was shut off during the peak hydraulic loading cycle. After 24 hours of operation at 6 gpm/ft2 without methanol feed, the hydraulic loading rate was reduced to 3.5 gpm/ft2 and methanol feed was re-established. This cycle was then repeated. Denitrification recovery during the peak hydraulic loading cycles are shown in Figure 10. The online monitoring data showed a rapid decrease in the effluent NOx-N as soon as methanol dosing was re-established. On March 5, the methanol dosing pump was started at 8:33 am and the hydraulic loading rate was decreased at 9:16 am. Between 9:00 and 9:30 the online monitoring data indicated that the effluent NOx-N concentration decreased from 4.32 to 1.03 mg/L. A similar rapid recovery of denitrification also was observed on March 8. The recovery of denitrification can be considered almost immediate.

Figure 10Recovery of Denitrification After Peak Hydraulic Loading Event

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DISCUSSION OF RESULTS Empty Bed Detention Time EBDT has historically been used as a design criterion for denitrification filters. In early work by Savage (1983), design curves were developed for conventional downflow denitrification filters relating EBDT to the percent NO3-N removed in the filter. The successful use of such curves is

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dependent in part by other factors (such as methanol dosing rates, influent DO and influent phosphorus) not being limiting. The pilot influent and effluent composite samples analyzed by the laboratory during operation at constant rate hydraulic loading were used to assess the impact of EBDT on percent NOx-N removal in the pilot filter and the effluent NOx-N concentration. Filter performance under diurnal flow variations was evaluated during Phase 4. The online nutrient monitoring data collected during this testing phase was used to examine the impact of transient changes in EBDT on NOx-N removal performance. Operation with diurnal variations at an average flow rate of 3.5 gpm/ft2 resulted in hourly hydraulic loading rates ranging from 2.2 to 4.375 gpm/ft2, or EBDTs ranging from 22.6 to 11.4 minutes, respectively. The impact of short-term variations on EBDT on NOx-N removal is shown in Figure 11.

Figure 11Transient Empty Bed Detention Time vs. Percent NOx-N Removed

and Effluent NOx-N Concentration

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Figure 11 indicates that under diurnal flow variations, operation at lower EBDTs (higher hydraulic loading rates) had a slight impact on the percent NOx-N removed. However, even under these transient loading conditions, very good NOx-N removals were maintained throughout the course of testing. It is noted that the effluent NO3-N and NO2-N readings from the online monitoring data trended slightly higher than the laboratory composite samples. The laboratory analyses were conducted in accordance with Standard Methods and are the values that would be used for compliance reporting.

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Nitrate Mass Loading During the pilot test, the filter was subjected to a wide range of nitrate mass loading conditions to assess the impact of nitrate loading on filter performance. Figure 12 shows the mass NOx-N loading and effluent NOx-N concentration during the Phase 2 testing at constant rate hydraulic loading. Although the hydraulic loading rate was constant, there was significant variation in the mass loading due to changes in the WWTP secondary effluent quality and the normal diurnal variations in concentration. As the hydraulic loading rate increased from 2 to 4 gpm/ft2, the average mass loading rates increased from about 20 to 40 lbs NOx-N /1,000 ft3. Throughout the testing virtually all of the NOx-N applied to the filter was denitrified.

Figure 12NOx Loading and Removal During Steady-State Operation

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To examine the short-term impact of the diurnal loading variations on pilot performance, an hourly time plot of the loading rate versus the effluent NOx-N concentration was developed (Figure 13). Although filter performance met the requirement of less than 1 mg/L NOx-N continuously, a slight increase in the effluent NOx-N concentration was observed during periods of higher loading. The increase in effluent nitrate appears to be impacted more by the mass loading rate than by the hydraulic loading rate.

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Figure 13Diurnal Variations in Mass Loading and Effluent Nitrate

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To further examine the short-term impact of the diurnal loading variations on pilot performance at the higher NOx-N loading rates, an hourly time plot of the loading rate versus the effluent NOx-N concentration was developed (Figure 14). This data represents operation at temperatures ranging from 13.9°C – 14.6°C and includes results from a period of testing in which sodium nitrate was added to the filter influent to increase the NOx-N loading rate.

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Figure 14Diurnal Variations during High Nitrate Loading

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These data show a stronger relationship between the mass loading rate and effluent quality. The change in loading conditions throughout the day clearly impacted the effluent quality. It is noted that once the filter reached the sustained peak loading rate in the afternoon of each day, the very low effluent nitrate concentrations observed earlier in the day at the low loading rates could not be achieved. This suggests that the process had reached the upper limit of treatment possible given the testing conditions (bed turnover, reject rate, etc.). The NOx-N mass loading rate vs. percent NOx-N removed is shown in Figure 15 and the NOx-N mass loading rate vs. effluent NOx-N is shown in Figure 16. At the winter wastewater temperatures tested, average NOx-N removals of 90 percent or greater could be maintained up to a NOx-N loading rate of 100 lbs/1,000 ft3/day. At higher loading rates the percent removal decreased and maintaining low effluent NOx-N concentrations became more difficult.

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Figure 15Impact of NOx-N Loading Rate on Percent NOx-N Removed

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Figure 16Impact of NOx-N Loading Rate on Effluent NOx-N Concentration

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Temperature Impact on Denitrification Temperature can have a significant impact on bacterial growth rates in biological wastewater treatment processes, with higher growth rates observed during summer operating conditions and lower growth rates observed during winter conditions. Since operation during cold weather would be the limiting condition for the denitrification filter design, the pilot testing at Hagerstown was scheduled for the November through March time period. During the pilot testing, the main WWTP effluent wastewater temperatures varied from 13.2 to 19.3oC based on the treatment plant effluent readings. The denitrification performance consistently met the effluent goal of less than 1 mg/L NOx-N at these temperatures. Phosphorus Limitation on Denitrification Phosphorus is required for cell growth and respiration and is therefore needed for growth of denitrifying bacteria in the filter. At Hagerstown, the denitrification filter is being considered as a “tertiary add-on” process, and will be located downstream from a biological phosphorus removal process. The BNR process effluent phosphorus concentration is often very low, with TP and OP concentrations of less than 0.5 and 0.02 mg/L, respectively. This is of concern because if the upstream processes remove too much phosphorus from the filter influent wastewater, denitrification in the filter could be adversely impacted. Therefore, a key consideration during pilot testing was to determine the minimum phosphorus requirement for denitrification. During Phase 5 of the pilot testing, additional nitrate was added to the filter influent wastewater to determine the nitrate removal capacity of the filter. During the first several days of phase 5, no additional phosphorus was added, and significant bleedthrough of NOx-N to the effluent resulted. During Phase 5b, phosphoric acid was added to increase the available phosphorus, and denitrification performance improved dramatically. This improved performance was also achieved at even higher nitrate loading rates during Phase 5c. These results are discussed in detail in deBarbadillo et al (2006). With the exception of the testing at high nitrate loadings, denitrification performance was excellent and did not appear to be impacted by the low phosphorus. To examine the impact of soluble phosphorus on denitrification performance, the ratio of influent OP to NOx-N was compared to the effluent NOx-N concentration. The results suggest that the low influent OP did not result in elevated NOx-N concentrations until the OP/NOx-N ratio was reduced to less than 0.02, which is much lower than the theoretical requirement of 0.04 g P/g NOx-N denitrified. Since the secondary effluent from the full scale Hagerstown WWTP contained phosphorus accumulating organisms, it is likely the balance of the phosphorus requirement was met by phosphorus released from the biomass. Methanol Feed During initial startup, a relatively high methanol dosing rate of over 60 mg/L was fed to the pilot filter to encourage the growth of denitrifying bacteria in the filter bed. After the effluent NOx-N concentration decreased to about 1 mg/L, the methanol feed rate was reduced and automatically

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dosed based on filter influent flow, NO3-N, NO2-N, and DO concentrations reported by the online instrumentation. Using the findings of McCarty, et al (1969) as guidance, weighting factors on NO3-N, NO2-N, and DO were selected. The methanol dosing parameters were adjusted several times through the course of the pilot testing in an effort to find the optimal dosing rate that was sufficient for full denitrification without increasing the effluent BOD concentration. Overall, the dosing rate was consistently less than 3 mg methanol per mg NOx-N removed. To assist in the evaluation of methanol dosing rates, a methanol dosing ratio was calculated using the actual methanol dosed divided by the NOx-N concentration in the influent stream. The methanol dosage ratio, effluent NOx-N and influent DO are shown in Figure 17. The dosing ratio has a diurnal pattern which reflects the diurnal variation in the influent DO concentration. As the multipliers for the feed forward control were decreased, the pilot continued to produce an effluent NOx-N of less than 1 mg/L.

Figure 17Methanol Dosing versus Effluent NOx

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Figure 17 shows that the effluent NOx-N concentration was consistently below the target effluent concentration of 1 mg/L from mid-December through January even at methanol/NOx-N ratios of 2.5 and lower. The reduced dosage ratios as calculated resulted in part from the downward trend in the filter influent DO through the course of testing, but these dosage rates generally were lower than expected. One objective of the pilot test was to determine the methanol dosing requirements to achieve full denitrification to effluent NOx-N concentrations of less than 1 mg/L

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without increasing effluent BOD5 from methanol breakthrough. To examine whether methanol was bleeding through to the effluent, filter influent and effluent BOD and COD values were compared and the change in BOD and COD across the filter were calculated. Although significant scatter is observed in the data, as methanol dosage ratios decreased, there was generally a net reduction in effluent BOD and COD. However, although the methanol dosages are very low, the slightly elevated BOD and COD concentrations in the effluent would be of concern for a WWTP with a very low effluent BOD limit. A high level of methanol control is recommended to ensure that the BOD limits are met. Solids Removal In addition to denitrification performance, solids removal in the filter is an important component for ensuring that anticipated permit limits would be met. The filter influent and effluent concentrations for the duration of pilot testing are shown in Figure 18. During operation of the pilot, the filter influent TSS concentrations ranged from 2 to 45 mg/L and averaged 9.7 mg/L. The corresponding effluent TSS concentrations ranged from 1.2 to 17.6 mg/L, and averaged 4.9 mg/L for the 18 weeks of testing. On a 30-day rolling average basis, the filter effluent solids ranged from 4 to 6 mg/L. All effluent solids concentrations consistently met the monthly average effluent TSS limit of 12 mg/L stipulated by the Hagerstown WWTP NPDES permit. For most of the duration of testing, the pilot was operated under the equivalent of a dry weather flow condition. Under these conditions, a month average TSS limit of 5 mg/L would be met. However, under conditions simulating peak day flows, the pilot slightly exceeded this limit. The filters would be expected to easily meet the Hagerstown effluent limit, but meeting very low solids concentrations of less than 5 mg/L possibly could be problematic at these loading rates.

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Figure 18Filter Solids Removal Throughout the Pilot Test

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To further evaluate the solids removal performance of the filter over the range of loading conditions, the solids loading rate (in lbs/ft2) vs. the effluent TSS concentration is shown in Figure 19. In general, at higher solids loading rates, elevated TSS concentrations were observed. Based on the results it is estimated that solids loading rates to the filters should be maintained at 0.5 lbs TSS/ft2/d or lower if an effluent TSS concentration of 5 mg/L or lower is to be maintained. This solids loading rate limitation is specific to denitrification mode under the conditions tested and at a media recirculation rate corresponding to 3 bed turnovers per day.

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Figure 19Filter Solids Loading Rate vs. Effluent TSS Concentration

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Filter Bed Turnover Rates Denitrification filter performance is dependent in part on successfully growing and maintaining denitrifying biomass within the filter. However, as biomass accumulates and as solids are removed from the filter influent wastewater, the filter must be backwashed. With upflow continuous backwash filters, this is accomplished by the continuous circulation of the filter media through the airlift and sand washer. The backwash frequency is quantified by the bed turnover rate. Typically, downflow denitrification filter installations backwash approximately once per day under dry weather design conditions. However, for typical applications for solids removal only, upflow continuous backwash filters are operated at four to six bed turnovers per day (or four to six backwashes per day). Therefore, it was expected that the bed turnover rate in the pilot would need to be reduced in order to retain sufficient biomass for denitrification. Based on physical measurements of sand movement, this corresponded to a bed turnover rate of 3.125-inches per hour or 1.06 bed turnovers per day based on 80 inches effective media depth. The bed turnover rate was adjusted several times during the pilot ranging from 1 to 3.3 bed turnovers per day. It is noted that the bed turnover rate was affected by the air flow and temperature fluctuations, but good denitrification performance was maintained at each of these

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turnover rates. A full scale installation should be designed to allow adjustment of the turnover rate as needed to maintain good denitrification and solids removal performance. Filter Headloss The headloss across the filter remained low throughout testing. Headloss was measured by both a differential pressure sensor and using a visual manometer that was recorded by the operators three times each day. The differential pressure sensor provided inaccurate measurements during extreme cold weather periods during the test because of freezing. Filter headloss recorded by the differential pressure sensor and influent flow measurements for the entire testing period of are presented in Figure 20. Extreme headloss readings above the mean daily averages were generally resulted from drops in air flow to the airlift at night due to drops in air temperature or periodic freezing of water in the instrument tubing periodically during January and February.

Figure 20Filter Headloss

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The headloss across the filter remained under 20 inches of water column (wc) throughout the test. The filter showed distinct increases in headloss each time the flow rate was changed. Although variations in headloss did occur, the graph indicates that headloss was a function of hydraulic loading and may be influenced to a lesser extent by solids loading and chemical feed for phosphorus removal. From March 8th through 17th, when primary clarifier effluent was blended with the feed, the headloss remained below 12 inches wc.

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KEY FINDINGS The denitrification filter met all of the pilot objectives, and should be a reliable process for upgrading the Hagerstown WWTP to meet ENR limits. Key conclusions are summarized below in several categories, including performance objectives, design criteria, chemical dosing and filter operating parameters. Performance Objectives

• The filter consistently achieved an effluent NOx-N concentration less than 1.0 mg/L under all conditions tested, including operation under constant hydraulic loading conditions and under diurnal flow and loading conditions that would be expected at the full-scale plant.

• Concurrently with meeting denitrification objectives, the filter consistently achieved an effluent TP of less than 0.3 mg/L with low dosages of ferric chloride.

• Occasional sudden increases in NOx-N from the upstream BNR process resulted in some temporary NO2-N breakthrough at the denitrification filter. This may occur occasionally at full scale, but the filter would still meet the overall performance objectives.

• The filter effluent BOD concentrations were consistently below the Hagerstown WWTP effluent limit of 12 mg//L monthly average.

• The filter effluent TSS concentrations averaged about 5 mg/L. Higher effluent TSS concentrations were observed under higher filter influent solids loading conditions, but the Hagerstown effluent TSS limit of 12 mg/L monthly average was consistently met.

Design Criteria

• The filter consistently achieved an effluent NOx-N concentration of less than 1 mg/L under constant hydraulic loading rates of 4.0 gpm/ft2 and peak hydraulic loading rates up to 6.0 gpm/ft2. Effluent criteria also were met under a diurnal flow pattern with an average hydraulic loading rate of 3.5 gpm/ft2.

• The filter performed well for an extended period of operation under average mass loading rates 30 to 40 lbs NOx-N/1000 ft3/d. This corresponds to range of mass loading rates observed under the diurnal loading pattern at an average hydraulic loading rate of 3.5 gpm/ft2 and typical filter influent NOx-N concentrations of 5 to 6 mg/L. The filter also performed well at higher mass loading rates of up to 100 lbs NOx-N/1000 ft3/day.

• Denitrification objectives were consistently met under sustained winter wastewater temperatures of 13 to 15 oC. Temperature fluctuation did not impact performance results under the range of conditions tested.

• To achieve effluent TSS concentrations of consistently less than 5 mg/L, the filter average solids loading rate should not exceed about 0.5 lbs TSS/ft2/d. This value is specific to operation in denitrification mode.

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Chemical Dosing Considerations

• Denitrification objectives were met with methanol to influent NO3-N ratios between 2.5 and 3.

• Phosphorus removal objectives were met with direct ferric chloride injection into the filter influent piping. Additional reaction time did not result in improved performance under the conditions tested.

• The online nutrient analyzer proved to be reliable and can be successfully used for feed-forward control of methanol and ferric chloride dosing under diurnal flow and concentration variations.

• The feed-forward methanol dosing scheme was adequate for meeting performance objectives with little or no methanol carryover into the filter effluent.

Filter Operating Parameters

• Denitrification performance was successfully maintained under media recirculation rates

of 1 to 3 bed turnovers per day. This rate is lower than the typical range of 4 to 6 bed turnovers per day used for solids removal applications.

• The headloss through the filter was successfully maintained at less than 2 ft throughout testing.

• The pilot operated with reject (backwash) rates of 6 to 12% of the forward flows, with a reject rate of about 9% observed at hydraulic loading rates of 3 to 3.5 gpm/ft2. Based on the typical full-scale airlift and filter media wash assembly in combination with the pilot results, it is estimated that a slightly lower average reject rate of 8% could be expected from a full-scale installation. Under peak flow operating conditions, the reject rate would be about 5%, and under low flow conditions, the anticipated full-scale reject rate would be about 10% of forward flow.

REFERENCES deBarbadillo, C., Rectanus, R.., Janssen, D., and B. Boyd (2006). Tertiary Denitrification and Low Phosphorus Limits: Phosphorus Release in Filters Helps Promote Adequate Denitrification, accepted for presentation at the International Association on Water Quality Biofilm Systems VI Conference, Amsterdam, The Netherlands, September 24 through 27, 2006. Hultman, B., Jonsson, K., and E. Plaza (1994). Combined Nitrogen and Phosphorus Removal in a Full-Scale Continuous Upflow Sand Filter, Wat. Sci. Tech., Vol. 29 (10/11), 127-134. Jonsson, L., Plaza, E., and B. Hultman (1997). Experiences of Nitrogen and Phosphorus Removal in Deep-Bed Filters in the Stockholm Area, Wat. Sci. Tech., Vol. 36, 183-190. Koopman, B., Stevens, C., and C. Wonderlick (1990). Denitrification in a Moving Bed Upflow Sand Filter, Res. J. Water Pollut. Control Fed., 62, 239-245

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Kramer, J.P., Wouters, J.W., and P. van Rosmalen (2003). Moving Bed Biofiltration for Dynamic Denitrification, Four Years of Operating Experience, Proceedings of the Water Environment Federation 76th Annual Conference, Los Angeles, CA. Luedecke, C., Hermanowicz, S.W. and Jenkins, D. Precipitation Of Ferric Phosphate In Activated Sludge: A Chemical Model And Its Verification, Wat. Sci. Tech., 21, 352, 1988 McCarty, P.L. and Amant, P.S. (1969). Biological Denitrification of Wastewaters by Addition of Organic Materials, Proceedings 24th Industrial Waste Conference, Purdue University, 1271 – 1285. Savage, E.S. (1983). Biological Denitrification Deep Bed Filters, presented at the Filtech Conference, Filtration Society, London, England. Water Environment Federation (1998). Special Publication: Biological and Chemical Systems for Nutrient Removal, Alexandria, Virginia.

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